Towards sustainable microalgal biomass processing: anaerobic induction of autolytic cell-wall self-ingestion in lipid-rich Nannochloropsis slurries

The high energy requirement associated with cell disruption has remained a major barrier to the commercial production of microalgal biofuels and bioproducts. Autolysis takes advantage of microalgal cells’ biologically driven self-lysing properties and represents an environmentally sustainable mean of improving the recovery of intracellular products and the energy economics of biomass processing. This study demonstrated for the first time the induction of autolytic cell-wall thinning in Nannochloropsis. Autolysis was induced using a low-cost and low-energy non-invasive incubation treatment which stored highly concentrated microalgal slurry (0.11–0.29 mg biomass per mg slurry) in darkness at 38 °C for 24 h. Under thermally coupled dark-anoxia incubation, the Nannochloropsis cells were deprived of oxygen and activated anaerobic metabolism that depleted intracellular sugar reserves to sustain basal metabolism (total sugar content of the biomass decreased from 0.174 ± 0.029 to 0.124 ± 0.027 mg mg−1 biomass). The auto-fermentation pathways consumed the polysaccharide reserves in the cell wall and resulted in a ∼50% reduction in the thickness of the cellulose layer of the cell wall (from 27.8 ± 9.4 nm to 12.6 ± 5.5 nm). This wall thinning effect structurally weakened the cells, significantly increased the percentage of cells that could subsequently be ruptured with a range of cell-disruption technologies, such as high-pressure homogenisation (HPH), alkaline treatment or acidic treatment (from 36.8 ± 11.9% for untreated slurry to 74.1 ± 14.2% for incubated slurry), and ultimately led to a 2-fold increase in the amount of lipid recovered with a low-solvent biphasic extraction system (from 22.2 ± 13.5 wt% of lipid for the untreated slurry to 48.6 ± 17.6 wt% of lipid for the incubated slurry).